1. Field of the Invention
This invention relates to a laser beam focusing apparatus and a method for focusing a laser beam, particularly usable for laser nuclear fusion and laser particle acceleration and induced nuclear reactions.
2. Description of the Prior Art
Recently, much attention is paid to such research and development as to generate nuclear fusion by heating instantaneously a ultra high density plasma (solid plasma or more dense plasma) using a short pulse high intensity laser and is also paid to various structural analyses using laser produced high energy particles.
Conventionally, such a high intensity laser beam is generated by focusing a laser beam by a focus lens system or mirror. In such a conventional means, however, it is difficult to focus the laser beam in high intensity because of the wavefront distortion of the laser beam, so a focused laser beam having a spot size of 10 xcexcm or below is not easy to be realized. In this point of view, such an attempt is made as to focus a laser beam by means of a complicated optical system with an optical waveguide, but the operationallity is deteriorated and the cost is increased because of the complicated optical system.
It is an object of the present invention, in this point of view, to provide a new apparatus and a new method whereby a laser beam of high intensity can be obtained through focusing a pulsed laser without a complicated optical system.
In order to achieve the above object, this invention relates to a laser beam focusing apparatus comprising a hollow tube having a pair of openings which are opposed each other,
one opening serving as a laser beam inlet,
the other opening serving as a laser beam outlet,
the diameter of the laser beam inlet being set larger than the diameter of the laser beam outlet.
Also, this invention relates to a method for focusing a laser beam, comprising the steps of:
preparing a hollow tube having a pair of openings which have their respective different diameters and are opposed each other,
introducing a pulsed laser beam into said hollow tube from the larger opening of said hollow tube
reflecting multiply said pulsed laser beam on the inner wall surface of said hollow tube and traveling said pulsed laser beam toward the smaller opening of said hollow tube with condensation, to generate a focused laser beam, and
outputting said focused laser beam from the smaller opening of said hollow tube.
The inventor had intensely studied to achieve the above object. As a result, the inventor found out the following fact. First of all, a hollow tube having a pair of openings which are opposed each other is prepared. Then, a given pulsed laser beam is introduced into the hollow tube from the larger opening. In this case, a plasma layer having a thickness much smaller than the wavelength of the laser beam is created on the inner wall of the hollow tube, and the introduced laser beam is reflected multiply on the inner wall of the hollow tube toward the smaller opening from the larger opening to generate a highly focused laser beam. Therefore, when the size of the smaller opening is set to a desired size, the pulsed laser beam can be focused to a diffraction-limited spot size of about 10 xcexcm and emitted outside.
According to the present invention. only if such a hollow tube having a pair of openings which have different sizes and are opposed each other, a laser beam of high intensity can be easily obtained.